In known apparatuses of this kind, a first fluid flows through the main heat exchanger, while a second fluid, which is cooled by the first fluid, flows through the secondary heat exchanger. In the application of such an apparatus to motor vehicles, the first fluid is the cooling liquid for the engine of the vehicle, while the second fluid may for example be a lubricating oil for the engine, or gearbox oil, or oil for a torque converter. Thus the engine coolant liquid not only fulfils its usual function of cooling the engine, but also an additional function whereby it cools a further fluid related to the operation of the vehicle.
In known arrangements of this kind, the secondary heat exchanger is introduced through an open side of the water box, and the latter is then joined to the body or the tube bundle of the main heat exchanger. The secondary heat exchanger has two coupling tubes, which serve respectively as input and output tubes for the second fluid, and which extend sealingly through two orifices formed in a wall of the water box. This arrangement also secures and retains the secondary heat exchanger within the water box.
The specification of French patent application No. 83 12200, published under the number FR-2 549 593-A, describes an apparatus of the kind described above, in which each of the two coupling tubes is fitted on the secondary heat exchanger by introducing it into one of the holes in the side wall of the water box, and is then secured to a base member of the secondary heat exchanger. In this known arrangement, each coupling tube includes a threaded portion which is screwed into a threaded bore of the base member. The latter is engaged against the inner face of the wall of the water box, with two seals being interposed between them. These seals are compressed under the action of the gripping force exerted by a nut which is screwed on to a further threaded portion of the coupling tube, and which bears in abutting engagement against the outer face of the water box side wall.
In this known arrangement, the threaded portion of the coupling tube must extend partly into the interior of the secondary heat exchanger, which must then have a width sufficient to accommodate this threaded portion. This leads to the use of a secondary heat exchanger which is somewhat large, and this in turn increases the size of the water box. In addition, since the threaded portion projects into the interior of the secondary heat exchanger, this disturbs the flow of fluid circulating in the latter. This known arrangement is also somewhat complicated to manufacture, in particular since it necessitates two thread forming operations for each of the two coupling tubes.